The present disclosure relates generally to gas recovery systems, and particularly to gas recovery systems including an electrochemical compressor.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell can function as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas, and can function as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity. Referring to FIG. 1, which is a partial section of a typical anode feed electrolysis cell 100, process water 102 is fed into cell 100 on the side of an oxygen electrode (anode) 116 to form oxygen gas 104, electrons, and hydrogen ions (protons) 106. The reaction is facilitated by the positive terminal of a power source 120 electrically connected to anode 116 and the negative terminal of power source 120 connected to a hydrogen electrode (cathode) 114. The oxygen gas 104 and a portion of the process water 108 exit the cell 100 as byproducts 109, while protons 106 and water 110 migrate across a proton exchange membrane 118 to cathode 114 where hydrogen gas 112 is produced.
Another typical water electrolysis cell using the same configuration as is shown in FIG. 1 is a cathode feed cell, wherein process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode where hydrogen ions and oxygen gas are formed due to the reaction facilitated by connection with a power source across the anode and cathode. A portion of the process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell uses the same general configuration as is shown in FIG. 1. Hydrogen, from hydrogen gas, methanol, or other hydrogen source, is introduced to the hydrogen electrode (the anode in fuel cells), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in fuel cells). Water can also be introduced with the feed gas. Hydrogen electrochemically reacts at the anode to produce protons and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water, which additionally includes any feed water that is dragged through the membrane to the cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.
In other embodiments, one or more electrochemical cells can be used within a system to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water as needed. Such systems are commonly referred to as regenerative fuel cell systems.
Electrochemical cell systems typically include a number of individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits or ports formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. The cathode and anode may be separate layers or may be integrally arranged with the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane-electrode assembly”, or “MEA”) typically has a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may furthermore be supported on both sides by screen packs or bipolar plates that are disposed within, or that alternatively define, the flow fields. Screen packs or bipolar plates may facilitate fluid movement to and from the MEA, membrane hydration, and may also provide mechanical support for the MEA.
In order to maintain intimate contact between cell components under a variety of operational conditions and over long time periods, uniform compression may be applied to the cell components. Pressure pads or other compression means are often employed to provide even compressive force from within the electrochemical cell.
It is often desired for an electrochemical cell system to deliver hydrogen absent water or moisture vapor. Dryers, including Pressure Swing Adsorption (PSA) units, are used to remove moisture from the hydrogen produced by the electrolysis cell. To maintain functionality, the PSA used in existing systems is regenerated, or purged with a slipstream of dry hydrogen. Because the slipstream purge, which can be about 5 to 15 percent of the hydrogen produced by the cell includes moisture, it is typically vented, or discarded. Accordingly, the electrolysis cell must produce about 5 to 15 percent more hydrogen than is actually delivered to an end user or end user system, reducing an overall productive efficiency of the electrochemical cell system. Additionally, it is desired to deliver the hydrogen with a pressure that is higher than that provided by the electrolysis cell. Mechanical compressors are often used to increase the pressure of the hydrogen. However, mechanical compressors consume power, and use moving parts that may require maintenance, thereby reducing an overall reliability and efficiency of the electrochemical cell system. Furthermore, mechanical compressors are not tolerant of moisture within the hydrogen. Accordingly, there is a need in the art for a hydrogen generation arrangement that overcomes these drawbacks.